1. Field of the Invention
The present invention relates to a process for producing a fiber and a method for producing a catalyst layer for a membrane/electrode assembly for a polymer electrolyte fuel cell.
2. Discussion of Background
As a process for forming a fiber, a process is known which comprises discharging a spinning solution from a discharge nozzle and evaporating a solvent of the spinning solution to form a fiber (Patent Document 1).
Further, as a polymer electrolyte fuel cell, one provided with a membrane/electrode assembly having a polymer electrolyte membrane disposed between two electrodes each having a catalyst layer, is known.
The polymer electrolyte fuel cell is required to provide a high energy efficiency and a high output density under an operation condition to provide high hydrogen and oxygen utilization rates.
In order for a polymer electrolyte fuel cell to satisfy such requirements, the gas diffusion in a catalyst layer is particularly important among key elements constituting the cell.
Conventional membrane/electrode assemblies are produced, for example, by the following methods.
(i) A method wherein a liquid mixture having a catalyst and a fluorinated ion exchange resin dissolved or dispersed in a solvent such as an alcohol (such as ethanol) is directly applied to the surface of a polymer electrolyte membrane to form a catalyst layer.
(ii) A method wherein such a liquid mixture is applied to a substrate film to form a catalyst layer, and such a catalyst layer is transferred or bonded to the surface of a polymer electrolyte membrane.
However, a membrane/electrode assembly obtainable by such a method is inadequate in the formation of pores in the catalyst layer, and the gas diffusion in the catalyst layer is low. Accordingly, when the polymer electrolyte fuel cell provided with such a membrane/electrode assembly is used at a high current density, the output voltage tends to decrease.
As a method for producing a membrane/electrode assembly having a catalyst layer having a high gas diffusion property, the following methods have been proposed.
(1) A method wherein from a liquid mixture having a catalyst and a fluorinated ion exchange resin dispersed or dissolved in a solvent, the solvent is removed to prepare particles having an average particle size of from 0.1 to 100 μm, such particles are applied on the surface of a polymer electrolyte membrane and press-bonded under heating to form a catalyst layer (Patent Document 2).
(2) A method wherein a catalyst and a fluorinated ion exchange resin are added to a solvent containing an alcohol (such as ethanol) and a fluorinated alcohol to prepare a liquid mixture, and a catalyst layer is formed by using such a liquid mixture (Patent Document 3).
However, even the catalyst layer formed by the method (1) or (2), was inadequate in the gas diffusion property. Further, the fluorinated alcohol is expensive as compared with a usual alcohol, and it must be recovered when it is evaporated by drying the coated film of the liquid mixture.
Patent Document 1: JP-A-2002-155421
Patent Document 2: JP-A-2001-185163
Patent Document 3: JP-A-2002-110202